Direct Experimental Evidence for Markedly Enhanced Surface Proton Activity Inherent to Water Ice

Autoionization and subsequent proton transfer processes determine the proton activity inherent to water molecular systems. In this study, we provide direct experimental evidence that the proton activity is markedly enhanced at the surface of crystalline ice, on the basis of the simultaneous observation of H/D exchange of water molecules at the surface and in the interior of well-defined double-layer ice films composed of H₂O and D₂O. Thermal desorption mass spectrometry showed clear signatures derived from the surface H/D exchange equilibrium, whereas infrared absorption spectroscopy indicated no appreciable H/D exchange progress in the interior. Detailed kinetic analyses revealed that the rate of H/D exchange at the surface is at least 3 orders of magnitude higher than in the interior. This drastic enhancement of the proton activity suggests an extremely high concentration of surface-hydrated protons in comparison with those in the bulk. Our results also highlight the impact of the local hydrogen-bond structure on the autoionization of water molecules.